hCaptcha - Do you use a captcha to keep out bots? hCaptcha is a drop-in replacement for reCAPTCHA that earns website owners money and helps companies get their data labeled.
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
AuthKong - A reliable captcha service, offers advanced security to protect websites from spam and abuse while ensuring a user-friendly experience for legitimate visitors.
MTcaptcha - Enterprise Captcha solutions
CyberSiara - Leverage the complexity of the human eye to beat bots in a way no other CAPTCHA can. Keep your privacy. Secure your Site. Enhance your User Experience.
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.